This invention relates to an axial piston machine and in particular it relates to an axial piston machine or pump including a drive shaft, a socket ring provided on the end face of the drive shaft, a rotary cylinder block having its axis of rotation inclined relative to the axis of the drive shaft, a plurality of pistons arranged for axial displacement in the cylinder block and being linked to the socket ring. The inclined position of the rotary cylinder block can be fixed or preferably can be made adjustable to control the displacement or strokes of the pistons.
The machines of this kind which are known as "Thoma" pumps, have a compact structure and by virtue of a complete hydraulic compensation of axial forces in the driving mechanism they are suitable for operation under extremely high pressures.
In conventional axial piston machines the inner face of the drive shaft is extended to form a socket ring for receiving ball-shaped ends of piston rods, thus forming universal ball-and-socket joints for the pistons. The opposite ends of the axial piston rods are tiltably linked to the pistons in the cylinder block. The cylinder block is rotatably supported in a barrel to rotate about an axis which intersects the axis of the drive shaft. To vary the strokes of the pistons, the barrel together with the block is tiltable about an axis which intersects the plane of the ball-and-socket joints of the socket ring on the drive shaft. In this plane is also situated the intersection point of the drive shaft axis with the cylinder barrel axis. By increasing the inclination of the cylinder block relative to the axis of the drive shaft, the stroke of the pistons is increased. Conventionally, the maximum angle of this inclination is about 27.degree..
Inasmuch as the piston rods are positively linked both to the pistons and to the drive shaft by universal joints, the movement of the pistons is enforced not only on the pressure side of the cylinder block when the pistons are forced outwardly by the pressure fluid, but also at the discharge or suction side when the pistons move inwardly.
For supporting the rotary part of such axial piston machines, especially in the case of axial piston pumps, radial and thrust roller bearings have almost without exception been employed for the drive shaft.
Due to the forceful and shock-like operation of the axial piston machines of this kind and also in view of the requirements for maximum adjustability of the tilting angle of the cylinder block relative to the drive shaft, the bearing strength, the wear resistance and the accuracy of the bearings for the drive shaft have to be extremely high.
Despite great effort to fulfill these requirements, the working life of such roller bearings is very short and this fact is only partially due to the wear of the roller bearing body. This wear depends to a great extent on the applied level of operational pressure of the machine and thus on the pressure load apart from the wear of the roller bearing itself. One of the most unpleasant side effects of the roller bearings is a high level of noise which is felt to be acutely disturbing. In addition, the roller bearings for the drive shaft due to their unavoidable play cause considerable irregularities in the rotary movement of the shaft which may amount to several tenths of a millimeter relative to the central axis of the bearing. Such an excessive aberration of the rotating shaft with respect to the center of the bearing is undesirable since excessive dynamic loads and vibrations of the driving mechanism necessitate additional tolerances in the high-pressure part of the machine in order to avoid excessive wear.
Many experiments have already been made to support the driving parts of Thoma pumps hydrostatically in order to attain in this manner a quite run of the shaft at a high wear resistance.
One of the largest problems encountered in hydrostatic bearings results from the pulsating loads transmitted from the varying number of pistons under load and such intermittent load condition necessitates sturdy and strongly damped bearings. Another problem is the susceptibility to the leakage of working fluid for the hydrostatic bearing support should be effected by an extremely thin oil film. The reduction of thickness of the bearing oil films requires, however, such a manufacturing accuracy of the parts of the bearing which in practical series production cannot be achieved.
Furthermore, not negligible temperature differences may occur between the drive shaft and the shaft bearing which due to the minute tolerances may result in a seizing tendency in the bearing.